Starting at 4:30, Morgan Stanley will host what it is calling an “open forum”—although sources say it is also cocktail party—honoring Mack, who is due to retire at the end of the year. Our sources say that attendance is expected to be so high that even a lot of Morgan Stanley people cannot get in.

There will, of course, be speeches by various folks—most likely including Mack himself and his successor, James Gorman. These will be broadcast on internal Morgan Stanley video feeds.

Mack retired as chief executive of Morgan Stanley on January 1, 2010.

Since then he has served as chairman of the board. Mack first joined Morgan Stanley in 1972. He rose to become a board member in 1987 and chief operating officer in 1993. He left Morgan Stanley in 2001, after a long power struggle with Phil Purcell following the merger with Dean Whitter.

He served as CEO of Credit SuisseFirst Boston before returning to Morgan Stanley as chief executive in 2005.

He was known affectionately—and, often, not so affectionately—as “Mack the Knife,” allegedly for his penchant for cost cutting. Others say the nickname came from his sometimes blood-thirsty approach to trading. One colorful tale—that may even be true—from the 1980s has him screaming over the trading floor, “There’s blood in the water. Let’s go kill!”

One of the funnier nicknames I heard a Morgan Stanley guy use was “Lady Mackbeth”—for Shakespeare’s lethal woman who always had blood on her hands.

Mack became one of the defining characters of Wall Street through the past few decades. We’ll raise a glass to him…and maybe even try to sneak into the “Open Forum.”